1. Field of the Invention
This invention relates to port output circuits and, more particularly, to apparatus for resetting the input/output ports of circuitry for switching high current digital signals.
2. History of The Prior Art
Digital circuitry is rapidly replacing mechanical devices and analog circuitry for controlling the operation of equipment. Digital circuits are typically small, operate rapidly, exhibit no signs of wear, and are inexpensive once in quantity production. However, many of the devices which are controlled require amounts of power to operate which are much larger than those typical to digital circuits. Consequently, it is often necessary to provide circuits which are capable of furnishing digital output signals which carry a great deal of current in order to provide the power necessary to operate other devices. Switching these high current output signals to output devices typically generates a high level of transient noise. These transients are often so large that they may interfere with the operation of, or actually damage, the digital circuitry providing the output signals, to circuitry to which the signals are furnished, or any digital circuitry which may be associated therewith.
To overcome this problem, digital buffer circuitry capable of switching high levels of current to an output without producing high levels of noise has been designed. Such circuitry in described in U.S. Pat. application Ser. No. 782,395, entitled ULRTA-LOW NOISE PORT OUTPUT DRIVER CIRCUIT, Hahn et al, filed Oct. 14, 1991, and assigned to the assignee of the present invention.
Not only are digital circuits useful for driving signals to an output, it may also be useful that those circuits respond to input signals generated external to the digital signal generators. To reduce the number of external terminals and for other reasons, it has been found desirable to design such circuits so that the output terminals may be used as input terminals for externally-generated input signals. In view of the fact that the output terminals may carry high current, it is very important that contention between the output signals generated within the digital signal generator and the input signals generated outside the digital signal generator be eliminated. Contentron between these signals might interfere with or even destroy the external circuitry.